Over-running output valves and rectifiers or allowing them to operate at extremely high envelope temperatures, can have another undesirable effect as the valve ages where electrolysis of the the envelope glass can occur.  The electrolytic action attacks the glass envelope in which lead (Pb) is extracted from the glass  as an anion which in turn is then attracted to internal wires having a negative charge resulting in metallic lead deposition around the negatively charged lead in wires -  especially where they enter the pinch or base button.   The effect of these 'Lead lead trees'  - which are often only of monolayer thickness - is to provide a leakage pathway which plays havoc with normal operation hence rendering the valve useless.